Expose top level Puma.stats API

Right now to get stats for a puma process the only way is to go through the control server. The puma-stats-logger does this by reading in a state file then querying the control server manually.

7ad7798e9d/lib/puma_stats_logger/middleware.rb (L28)

Instead, I’m proposing adding a top level `Puma.stats` method that will allow anyone inside of the same process to get access to the stats.

This could be instrumented by a gem to theoretically export these stats to say a Heroku dashboard where we could list out backlog or thread count.

The format of stats is a hash, and will change depending on if the server is in “single” or “clustered” mode.

Clustered:

```
{ "workers": 2, "phase": 0, "booted_workers": 2, "old_workers": 0, "worker_status": [{ "pid": 19832, "index": 0, "phase": 0, "booted": true, "last_checkin": "2018-03-12T16:03:12Z", "last_status": { "backlog":0, "running":5 } },{ "pid": 19833, "index": 1, "phase": 0, "booted": true, "last_checkin": "2018-03-12T16:03:12Z", "last_status": { "backlog":0, "running":5 } }] }
```

Single:

```
{ "backlog": 0, "running": 2 }
```

Alternatively if we could somehow enable another process to get these stats from Puma via pumactl by default without requiring any additional in app config, that would also work.

lib/puma.rb

@@ -12,4 +12,12 @@ module Puma
   autoload :Const, 'puma/const'
   autoload :Server, 'puma/server'
   autoload :Launcher, 'puma/launcher'
+
+  def self.stats_object=(val)
+    @get_stats = val
+  end
+
+  def self.stats
+    @get_stats.stats
+  end
 end

lib/puma/cli.rb

@@ -1,6 +1,7 @@
 require 'optparse'
 require 'uri'
 
+require 'puma'
 require 'puma/configuration'
 require 'puma/launcher'
 require 'puma/const'

lib/puma/launcher.rb

@@ -86,6 +86,7 @@ module Puma
       else
         @runner = Single.new(self, @events)
       end
+      Puma.stats_object = @runner
 
       @status = :run
     end

test/test_cli.rb

@@ -57,6 +57,7 @@ class TestCLI < Minitest::Test
     s << "GET /stats HTTP/1.0\r\n\r\n"
     body = s.read
     assert_equal '{ "backlog": 0, "running": 0 }', body.split(/\r?\n/).last
+    assert_equal '{ "backlog": 0, "running": 0 }', Puma.stats
 
     cli.launcher.stop
     t.join